The reason you can't invoice against Consumables for a participant is due to the Service Contract not having the Consumables category budget.

 If you click New in the Plan Budget table, you can add Consumables as well as the budget amount. When you do this, you may need to adjust Daily Life or ASCP. An example is you might take $1000 from ASCP in the Core Budget to use for the Consumable budget. Careview is designed to enable you to adjust Core Category budgets as needed.